GLYCOGEN METABOLISM DR. A. TARAB DEPT. OF BIOCHEMISTRY HKMU

OVERVIEW A constant source of blood glucose is an absolute requirement for human life Glucose is the greatly preferred energy source for the brain, and the required energy source for cells with few or no mitochondria, such as mature erythrocytes Glucose is also essential as an energy source for exercising muscle, where it is the substrate for anaerobic glycolysis

Blood glucose can be obtained from three primary sources – the diet, degradation of glycogen and gluconeogenesis Dietary intake of glucose is sporadic, and depending on the diet, is not always a reliable source of blood glucose In contrast, gluconeogenesis can provide sustained synthesis of glucose, but it is somewhat slow in responding to a falling blood glucose level

Therefore, the body has developed mechanisms for storing a supply of glucose in a rapidly mobilizable form, namely, glycogen In the absence of dietary source of glucose, this compound is rapidly released from liver and kidney glycogen Similarly, muscle glycogen is extensively degraded in exercising muscle to provide that tissue with an important energy source

When glycogen stores are depleted, specific tissues synthesize glucose de novo, using amino acids from the body’s proteins as a primary source of carbons for the gluconeogenic pathway

STRUCTURE AND FUNCTION OF GLYCOGEN Function of glycogen: The function of muscle glycogen is to serve as a fuel reserve for the synthesis of ATP during muscle contraction That of liver glycogen is to maintain the blood glucose concentration, particularly during the early stages of a fast Structure of glycogen: It is a branched-chain homopolysaccharide made exclusively from α-D-glucose

The primary glucosidic bond is an α(1→4) linkage After an average of eight to ten glucosyl residue, there is a branch containing an α(1→6) linkage

Glycogen structure

Fluctuation of glycogen stores: Liver glycogen stores increase during the well- fed state, and are depleted during a fast Muscle glycogen is not affected by short periods of fasting (a few days) and is only moderately decreased in prolonged fasting (weeks) Muscle glycogen is synthesized to replenish muscle stores after they have been depleted, for example, following strenuous exercise

SYNTHESIS OF GLYCOGEN (GLYCOGENESIS) Glycogen is synthesized from molecules of α-D- glucose The process occurs in the cytosol, and requires energy supplied by ATP and uridine triphosphate (UTP) A. Synthesis of UDP-glucose α-D-glucose attached to uridine diphosphate (UDP) is the source of all of the glycosyl residues that are added to the growing glycogen molecule

Uridine diphosphate glucose UDP-glucose

UDP-glucose is synthesized from glucose 1- phosphate and UTP by UDP-glucose pyrophosphorylase B. Synthesis of a primer to initial glycogen synthesis: Glycogen synthase is responsible for making the α(1→4) linkages in glycogen This enzyme cannot initiate chain synthesis using free glucose as an acceptor of a molecule of glucose from UDP-glucose

Addition of glucose to glycogen

Instead, it can only elongate already existing chains of glucose Therefore, a fragment of glycogen can serve as a primer in cells whose glycogen stores are not totally depleted In the absence of a glycogen fragment, a protein called glycogenin, can serve as an acceptor of glucose residues The side chain hydroxyl group of a specific tyrosine serves as the site at which the initial glucosyl unit is attached

A glycosidic bond is formed between the anomeric C1 of the glucose moiety derived from UDP-glucose and the hydroxyl oxygen of a tyrosine side-chain of Glycogenin. UDP is released as a product.

Cross section of glycogen molecule The component labeled G is glycogenin

Transfer of the first few molecules of glucose from UDP-glucose to glycogenin, is catalyzed by glycogenin itself, which can then transfer additional glycosyl units to the growing α(1→4)-linked glycosyl chain This short chain serves as an acceptor of glucose residues

C. Elongation of glycogen chains by glycogen synthase: Elongation of a glycogen chain involves the transfer of glucose from UDP-glucose to the nonreducing end of the growing chain, forming a new glycosidic bond between the anomeric hydroxyl of carbon 1 of the activated glucose and carbon 4 of the accepting glucosyl residue The enzyme responsible for making the α(1→4) linkages in glycogen is glycogen synthase

Glycogen synthase reaction

D. Forming of branches in glycogen: If no other synthetic enzyme acted on the chain, the resulting structure would be a linear (unbranched) molecule of glucosyl residues attached by α(1-4) linkages Such a compound is found in plant tissues, and is called amylose In contrast, glycogen has branches located, on average, eight glucosyl residues apart, resulting in a highly branched tree like structure that is far more soluble than the unbranched amylose

Branching also increases the number of nonreducing ends to which new glucosyl residues can be added, thereby greatly accelerating the rate at which glycogen synthesis and degradation can occur, and dramatically increasing the size of the molecule

1. Synthesis of branches: Branches are made by the action of the “branching enzyme”, amylo-α(1→4)→α(1→6)- transglucosidase This enzyme transfers a chain of five to eight glucosyl residues from the nonreducing end of the glycogen chain [breaking an α(1→4) bond] to another residue on the chain and attaches it by an α(1→6) linkage

Glycogen branching activity

The resulting new nonreducing end from which the five to eight residues were removed, can now be further elongated by glycogen synthase 2. Synthesis of additional branches: After elongation of these two ends has been accomplished by glycogen synthase, their terminal five to eight glucosyl residues can be removed and used to make further branches

DEGRADATION OF GLYCOGEN (GLYCOGENOLYSIS) The degradative pathway that mobilizes stored glycogen in liver and skeletal muscle is not a reversal of the synthetic reactions Instead, a separate set of cytosolic enzymes is required When glycogen is degraded, the primary product is glucose 1-phosphate, obtained by breaking α(1→4) glycosidic bonds In addition, free glucose is released from each α(1→6)-linked glucosyl residue

A. Shortening of chains: Glycogen phosphorylase sequentially cleaves the α(1→4) glycosidic bonds between the glucosyl residues at the nonreducing ends of the glycogen chains by simple phosphorolysis until four glucosyl units remain on each chain before a branch point The resulting structure is called a limit dextrin and phosphorylase cannot degrade it any further

Phosphorylase reaction

B. Removal of branches: Branches are removed by two enzymic activities First oligo-α(1→4)→α(1→4)-glucantransferase removes the outer three of the four glucosyl residues attached to a branch It next transfers them to the non-reducing end of another chain, lengthening it accordingly Thus an α(1→4) bond is broken and an α(1→4) bond is made

Next, the remaining single glucose residue attached in an α(1→6) linkage is removed hydrolytically by amlo-α(1→6)-glucosidase activity, releasing free glucose The glucosyl chain is now available again for degradation by glycogen phosphorylase until four glycosyl units from the next branch are reached

Glycogen debranching activity

Glycogen remodeling

C. Conversion of glucose 1-phosphate to glucose 6-phosphate: Glucose 1-phosphate, produced by glycogen phosphorylase, is converted in the cytosol to glucose 6-phosphate by phosphoglucomutase – a reaction that produces G1,6BP as a temporary but essential intermediate In the liver, G6P is translocated into the endoplasmic reticulum (ER) by glucose 6- phosphate translocase

Phosphoglucomutase reaction

There, it is converted to glucose by glucose 6- phosphatase – the same enzyme used in the last step of gluconeogenesis The resulting glucose is then transported out of the ER to the cytosol Hepatocytes release glycogen-derived glucose into the blood to help maintain blood glucose levels until the gluconeogenic pathway is actively producing glucose

*Note: - In the muscle G6P cannot be dephosphorylated because of a lack of glucose 6-phosphatase Instead, it enters glycolysis, providing energy needed for muscle contraction D. Lysosomal degradation of glycogen: A small amount of glycogen is continuously degraded by the lysosomal enzyme, α(1→4)- glucosidase (acid maltase)

The purpose of this pathway is unknown However, a deficiency of this enzyme causes accumulation of glycogen in vacuoles in cytosol, resulting in the serious glycogen storage disease type II (Pompe disease)

Glycogen-engorged lysosome This electron micrograph shows skeletal muscle from an infant with type II glycogen-storage disease (Pompe disease)